(1) Field of the Invention
This invention relates to a metallic slug for expulsion from an industrial ballistic tool. More particularly, it relates to a cost-efficient, environmentally friendly, frangible slug.
(2) Description of the Related Art
Industrial ballistic tools are used in a variety of applications. One common application is the in situ cleaning of kilns, for which the tools are commonly identified as kiln guns. Additional applications lie in the tapping and cleaning of furnaces, the cleaning of copper smelters, the cleaning and clearing of silos, the cleaning of boilers, and the like.
By way of example, rotary kilns, which are used to calcine cement and lime, are typically 3 to 7 meters in diameter and 30 to 150 meters long. Calcining takes place at elevated temperatures, typically in the range of 1100xc2x0 C. to 1500xc2x0 C. During the calcining process, because of many processing variables, the product may adhere to the sidewall of the kiln forming a clinker, ring or dam. If this adherent obstruction is not removed, additional product will accumulate, reducing or stopping throughput. Removal of the obstruction is necessary.
It is not economically feasible to stop the kiln to remove the obstruction. Also, considering that the ring may form 5 to 10 meters from the end of the kiln, it is not safe or efficient for an operator to attempt to manually remove the obstruction with a long pole or by like methods. Thus many users of rotary kilns utilize industrial ballistic tools. A tool operator will position the tool in a kiln port and then fire metallic projectiles at the obstruction. Impact of the projectiles with the obstruction removes the obstruction from the sidewall of the kiln.
The metallic projectiles are usually formed from lead, a dense material with a relatively low vaporization (boiling) temperature of 1750xc2x0 C. The lead projectiles knock clinkers from the kiln sidewall and then fall into the kiln and may be vaporized.
Industrial ballistic tools are also utilized by manufacturers of steel, ferrosilicon and other materials. Prior to casting these metals, molten metal is typically contained within an electric furnace sealed by a carbon or clay base plug. Since the molten metal is at a temperature in excess of 2500xc2x0 C., manual removal of the plug is not feasible. One way that the plug may be removed is with an industrial ballistic tool. A metallic projectile is fired from the industrial ballistic tool to break open the plug, starting the flow of molten metal. To prevent contamination of the metal, the projectile typically is formed of a material such as lead that will vaporize on contact with the molten metal after rupturing the plug. Due to environmental concerns, lead is being phased out as a projectile material for use with industrial ballistic tools. By way of comparison, the use of an exemplary 85 gram lead slug in a kiln or furnace application would introduce up to 85 grams of lead into the atmosphere. Prior to its removal from the U.S. market, a gallon (3.791) of leaded gasoline would contain approximately 0.1 grams of lead. Thus each lead slug represents the equivalent of about 3,000 liters (850 gallons) of such leaded gasoline. With the necessity to use many hundreds of slugs per day in certain kiln applications, the amount of lead involved can be significant.
Several substitutes have, to date, proven unsatisfactory. Iron and steel are much harder than lead, causing cast or forged iron or steel-based projectiles to be prone to excessive penetration and ricochet, potentially damaging the kiln and/or injuring the operator. U.S. Pat. No. 3,232,233 of Arthur Singleton discloses iron-based industrial slugs. The slugs are compacted and then sintered at a high temperature. An exemplary such slug is pressed at 414 MPa (30 tons per square inch (tsi) (60,000 psi)) and sintered at a temperature of 982xc2x0 C. (1800xc2x0 F.) for a minimum of 45 minutes. To facilitate fragmentation of the slug, it is optionally provided with a compartment or xe2x80x9ccavityxe2x80x9d to provide a rupture plane. The provision of such cavities adds additional manufacturing complexities and reduces the mass associated with a given overall size or envelope of a projectile.
Zinc and zinc alloys have also been utilized as lead substitutes. Their relatively low density may make them disadvantageous for certain uses. A ballistically stabilized zinc-based projectile is described in U.S. Pat. No. 5,824,944 of Jack D. Dippold et al.
Due to the phasing out of lead-based projectiles, there remains a need for a non-lead-based metallic projectile for use with industrial ballistic tools that does not suffer from the above-stated disadvantages.
Accordingly, it is an object of the invention to provide metallic projectiles for expulsion from an industrial ballistic tool effective to remove clinkers from kilns and/or carbon or clay plugs from electric furnaces.
In one aspect the invention is directed to a method for manufacturing a frangible industrial slug. A mixture of powders is provided having a composition that consists essentially of up to 35% ferrotungsten in particulate form, up to 3% lubricant, and the balance iron in particulate form with inevitable impurities. The mixture is compacted at a pressure of between about 138 MPa (20,000 psi) and about 827 MPa (120,000 psi) to form a compact. The compact is optionally sintered at a temperature no greater than about 900xc2x0 C.
In another aspect, the invention is directed to a frangible projectile for expelling from an industrial ballistic tool. A projectile consists essentially of a slug which consists essentially of a compacted and sintered material comprising up to 35% ferrotungsten, up to 3% lubricant and the balance iron with inevitable impurities. Frangibility is preferably achieved without the need for frangibility-enhancing bores and compartments, thus not compromising projectile mass and providing a frangibility characterized more by pulverization than by fragmentation. As distinguished from the residual porosity which may be inherent in a powder metallurgical process, such bores and compartments are deliberately placed (such as by machining or molding) and dimensioned to substantially increase frangibility.
In various embodiments of the invention, the ferrotungsten powder may have a particle size distribution such that at least about 40% of such powder can pass through a 100 mesh sieve having a characteristic opening of 0.15 mm. The iron powder may have a particle size distribution such that at least 80% can pass through the sieve. Preferably all of the iron powder can pass through a second 60 mesh sieve having a characteristic opening of 0.25 mm. In various embodiments, the iron powder may have a particle size distribution such that at least about 85% can pass through a 100 mesh sieve. In various embodiments, from 20 to 25% of the iron powder can pass through a sieve having the characteristic opening of 0.045 mm.
Advantageously, the compacting is performed at a pressure effective to provide the compact with a transverse rupture strength in excess of 5.5 MPa (800 psi), and, more preferably, in excess of 7.24 MPa (1050 psi). In various embodiments, the sintering of the compact is performed for a sintering time of from about 1 minute to about 2 hours at a sintering temperature of about 500xc2x0 C. to 900xc2x0 C.
Preferably the compacting and optional sintering are effective to provide the slug with sufficient frangibility that, when the slug is expelled from the tool at a muzzle velocity of 640-700 m/s (2100-2400 fps) and normally impacted with a non-armor steel plate having a yield strength of about 310 MPa (45,000 psi) at a distance of about 16 m (53 ft.) from the muzzle, on average a largest residual piece of the slug represents less than 70% of the slug mass and at least 25% of the slug mass is represented by pieces which pass through a 0.084 cm (0.033 inch) sieve. In various embodiments, similar properties may be desired when the muzzle kinetic energy is between about 9,500 N-m (7,000 ft.-lbs.) and about 10,400 N-m (7,700 ft.-lbs.), and the slug is fired from a distance of about 3 meters to about 20 meters.
High degrees of pulverization and minimizing the size of the largest residual piece are desirable. In various embodiments, the largest residual piece may be no more than 5% of the slug mass while the slug is substantially pulverized. In various embodiments, the largest residual piece may be no more than 50% of the slug mass and at least 40% of the slug mass is represented by pieces which pass through a 0.084 cm (0.033 inch) sieve.
Preferably the slug is dimensioned to be expelled from an 8-gauge tool. In various embodiments, such a slug may have a weight of between about 42.5 g (1.5 oz.) and about 65.2 g (2.3 oz.). More preferably, the weight may be between about 48.2 g (1.7 oz.) and about 59.5 g. (2.1 oz.). The material may preferably have a density of between 5.6 and 6.2 g/cc and, more preferably between 5.8 and 6.0 g/cc. In certain embodiments, when a slug is drop weight tested throughout a range of energies between 40 percent and 80 percent of 11,400 N-m, a largest intact residual piece of said slug typically constitutes no more than 70 percent of the slug mass.
Among the advantages of the invention is the provision of a slug which reduces or eliminates the introduction of toxic pollutants (e.g., lead) into the atmosphere. The invention further facilitates the provision of such a slug having sufficient mass, momentum, and kinetic energy when expelled from an industrial ballistic tool to perform effectively in a particular industrial application. The invention further facilitates the provision of the slug having a desired degree of frangibility, such frangibility effective to avoid ricochet and avoid significant damage to the surface of the kiln, furnace, silo or the like at which the expelled slug is directed. The metallic projectile may optionally include a relatively soft sleeve suitable for engaging the rifling of a ballistic tool barrel extension.
Projectiles with the high degree of frangibility facilitated by the present invention may find use in a variety of industrial applications for which conventional industrial slugs may not be advantageous. Where the frangibility allows the projectile to be largely pulverized upon impact (rather than merely fragmented into a modest number of discrete pieces), risk of ricochet is reduced and the projectiles may be useful over a wide range of angles of incidence.
An exemplary application involves the cleaning of accumulations from ladles used in the steel industry. In such an application a slug with insufficient frangibility may hit the ladle at a rather low angle of incidence and may be redirected by the ladle potentially risking injury to personnel and damage to equipment.
Another example involves the clearing of screens used in the mining industry. In the mining industry, heavy screens are often used to block large pieces of material (typically rock) from damaging equipment. In one exemplary situation, a loader is used to deliver material to a crusher which may be located at the bottom of a hole or pit. The loader drops the material into the hole whereupon the material encounters a screen. Small pieces of material fall through the screen while larger pieces remain atop the screen. An exemplary screen is formed of steel bars having an approximate 8xc3x9713 cm (3xc3x975 inch) cross-section and arrayed in a mesh defining holes approximately 36xc3x9736 cm (14xc3x9714 inches). The pieces which are small enough to fall through the screen are then crushed in the crusher and may be delivered back up to the opening of the hole via a conveyor. Instead of the prior practice of lowering a worker into the pit to manually break-up the pieces trapped by the screen, the worker may use an industrial ballistic tool located proximate the opening of the hole to break-up the trapped pieces by impacting them with industrial projectiles.
These and other aspects of the present invention will be readily apparent upon reading the following detailed description of the invention, as well as the drawing and the claims.